1. Field of the Invention
The present invention relates to a key input device identifying a key used to make an input, out of a plurality of keys, and particularly relates to a technique for enabling multiple key inputs.
2. Description of the Background Art
An electrical appliance, which operates in accordance with user's manipulations, generally has an input device for accepting a manipulation command. In many of the electrical appliances, a key (e.g. a manipulation button, a jog dial or the like) configured such that it can receive user's manipulations (e.g. a pressing force) is arranged at its panel surface, along with a display unit for displaying an operational state. In a multifunctional electrical appliance, in particular, there are arranged a number of keys corresponding to the number of functions. Accordingly, it is not uncommon that a few tens to a hundred (hundreds) of keys are arranged at the panel surface, for example, of the electrical appliance.
In order to cause the electric appliance to execute an operation in accordance with a key used to make an input, it is necessary to identify which key is used to make an input, out of such multiple keys. As one of the configurations identifying the key used to make an input, there has been known a configuration that generates a voltage value corresponding to a key used to make an input, by using a resistor string made of a plurality of resistors connected in series, and identifies the key based on the generated voltage value. For example, Japanese Utility Model Laying-Open No. 03-009036 discloses a circuit for inputting therein a plurality of pieces of switch information of a microprocessor system. This circuit has at least two lock switches and resistors connected in series, and the resistors are configured to be separated by the lock switches. A piece of switch information input independently is identified by appropriately selecting a resistance value of each of the resistors.
According to such a configuration as disclosed in Japanese Utility Model Laying-Open No. 03-009036, it is necessary to convert the generated voltage value into a digital signal by using an analog to digital converter, in order to cause a microprocessor system (CPU) to execute processing in accordance with the key used to make an input.
Accordingly, the resistor string must be configured to generate a voltage change larger than a resolution of the analog to digital converter in accordance with a key input. However, it is difficult to excessively raise a power supply potential, and hence the number of keys to be assigned to one resistor string is inevitably limited.
It is thus necessary to increase the number of resistor strings with the increase in number of keys to be arranged. At the same time, it is also necessary to increase the number of analog to digital converters corresponding thereto. However, the analog to digital converter is relatively large, which leads to a problem of increase in circuit size and electric power consumption as the number of keys to be arranged, namely, the number of analog to digital converters is increased.